1. Field of the Invention
The present invention relates to a metal-made carrier body for carrying thereon an exhaust gas cleaning catalyst which is generally provided as an exhaust gas cleaning means at an intermediate point of an exhaust system of an automobile.
More particularly, the invention relates to a metal-made carrier body for carrying thereon an exhaust gas cleaning catalyst, in which the individual components of the carrier body are joined with improved strength so as to resist against peeling due to thermal strains. The invention is also concerned with a method of manufacturing such metal-made carrier body.
2. Description of the Related Art
Conventional metal-made carrier bodies of the above sort, which are adapted to carry an exhaust gas cleaning catalyst thereon, include those having a structure formed by laminating at least one sheet like metal band and at least one corrugated metal band in layers or rolling them together into a multi-layered composite body with a view toward increasing the carrying area per unit volume, namely, with a view toward increasing as much as possible the effective area of contact between exhaust gas and the exhaust gas cleaning catalyst per unit volume and further reducing the weight of the metal-made carrier body to a maximum possible extent. As it is in the form of a honeycomb, this structure is hereinafter called "honeycomb core structure".
For example, a sheet-like metal band made of a heat-resistant thin metal sheet having a thickness of 0.1 mm or smaller and a corrugated band made from another thin metal sheet of the same type are superposed one over the other to have areas of contact therebetween. They are then rolled together spirally into a honeycomb core structure defining many network-patterned gas flow passages along a central axis thereof for allowing exhaust gas to pass therethrough. The honeycomb core structure is enclosed within a tubular metal casing which has a single-layer structure and opens in opposite ends thereof. The members of the honeycomb core structure, i.e., the sheet-like band and corrugated band are put together into a vibration-proof structure. Namely, the sheet like band and corrugated band as well as the honeycomb core structure thus rolled and the metal casing are joined together at the areas of contact therebetween by welding, brazing or the like. These joining means at the areas of contact between the honeycomb core structure and the metal casing are exemplified by the following:
(i) Electron beam welding, laser beam welding, arc welding, etc. as disclosed in Japanese Patent Laid-Open Publication No. 13462/1979 and Japanese Patent Laid-Open Publication No. 1585/1982, for example. PA1 (ii) Brazing, as disclosed in Japanese patent Laid-Open Publication No. 199574/1986. This brazing includes enclosing a honeycomb core structure in a metal casing, applying a brazing material over end portions of such a composite structure such as by painting or spraying, and heating around end portions of the composite structure.
The above conventional joining methods cause the following production and quality problems:
With the joining method of (i) above, it is necessary to use special and expensive equipments.
In the joining method of (ii) above, the applying of wax is laborious and time-consuming, and the degree of joining strength at the central portion of the areas of contact between the honeycomb core structure and the metal casing is not adequate.
Generally, the metal-made carrier body of this type is used in severe thermal environments and hence requires adequate durability against any breakage, cracking or peeling of the members making up the carrier body. In the exhaust system of an automobile, the members of the carrier body are subjected to strains (hereinafter called "thermal strains") due to the large difference between their coefficients of thermal expansion (volume expansion and linear expansion) under the severe cooling and heating cycle of from -20.degree. C. to 900.degree. C. These members are also subjected to vibrations from the automobile body. As a result, the members would be separated, cracked or otherwise damaged.
If different kinds of metal materials, i.e., austenitic stainless steel and ferritic stainless steel are used for the metal casing and the honeycomb core structure, respectively, in view of oxidation resistivity and thermal stability, the extent of separation of these two parts would be much greater because of the difference in thermal expansion between the two parts. If a small scale separation occurs at the areas of contact between the metal casing and the honeycomb core structure, such small separation would expand in relays. Concurrently with this separation, vibration from the automobile body is applied to these two parts so that especially the contact portions between a sheet-like band and a corrugated band constituting the honeycomb core structure are subjected to peeling, cracking, breakage or other damage.
Consequently, in the metal-made carrier body of this conventional type, the inner wall surface of the metal casing and the outer peripheral surface of the honeycomb core structure must be joined to each other reliably with adequate firmness. Heretofore, there has been proposed no effective joining method.